1. Field of the Invention
The present invention relates to integrated operational transconductance amplifiers.
2. Description of the Prior Art
Integrated continuous-time filters using bipolar complementary metal oxide semiconductor (BICMOS) operational transconductance amplifiers (OTAs) are known. A prior art operational transconductance amplifier (OTA) is shown in FIG. 4. In this OTA design the transconductance GM is set by the ratio of two currents. In this circuit GM=Itune/(R0*Ibias) and Itune is adjusted to change the GM of this OTA. Matching of the GM of multiple OTAs is set by the matching of current sources Itune required for each OTA. Matching of current sources is difficult due to typical analog circuit matching errors. Another problem of this prior art arrangement is that the signal path is susceptible to Miller multiplication of the capacitance at the JOUTP/JOUTN pins by way of the Q0/Q1 differential pair.
Prior art common mode control schemes tend to require a high power supply voltage and do not accurately control the common mode voltage at the high impedance nodes JOUTP and JOUTN in FIG. 4. Another problem with some prior art circuits is that the adjustment of the transconductance GM interacts with the common mode voltage level control.
A need exists for an OTA that facilitates more efficient adjustment of GM than known prior art arrangements. It is desirable to provide a continuous-time filter section using OTAs with accurate, effective and reproducible responses.